1. Field of the Invention
The present invention relates to apparatus for holding objects and, more particularly, to an apparatus for holding an optical element in an exposure apparatus.
2. Description of the Related Art
Projection exposure apparatuses are generally used to transfer a circuit pattern formed on a reticle (or a mask) onto a wafer or the like using a projection optical system in a manufacturing process for a semiconductor device, such as a semiconductor memory or a logic circuit, using photolithography.
The projection optical system includes optical elements, such as a mirror and a lens, which must be held with high accuracy.
FIG. 21 shows a holding apparatus described in Japanese Patent Laid-Open No. 2004-078209. Referring to FIG. 21, a holding apparatus 200A includes three holding units disposed at three positions along the outer periphery of an optical element 210A. Each of the holding units includes a held member 212A disposed on the outer periphery of the optical element 210A and a holding member 220A. The held member 212A has a V-shaped projection and the holding member 220A includes two spherical elements 222A and 224A. The held member 212A is held by the holding member 220A such that surfaces of the projection are in contact with the respective spherical elements. Thus, the optical element is held such that each of the holding units has two contact points. Therefore, the position of the optical element is regulated in two axial directions for each holding member 220A. Since three holding members are disposed at three positions, the position of the optical element 210A is regulated in six axial directions by the holding apparatus 200A as a whole. In other words, the optical element 210A is kinematically supported. In addition, the holding apparatus 200A includes parallel leaf springs 220 movable in a radial direction of the optical element 210A. Accordingly, even when the ambient temperature around the optical element 210A varies, the holding apparatus 200A allows thermal expansion of the optical element 210A while preventing the center O of the optical element 210A from being displaced from a light axis.
The repeatability of the surface shape of the optical element is an important factor for reducing aberrations in an optical system in which the optical element held by the holding apparatus is installed.
For example, the optical element is removed as necessary from the holding apparatus for cleaning or maintenance for damage, and then is reattached to the holding apparatus. It is known that the surface shape of the reattached optical element differs from the original surface shape.
In the structure of the above-mentioned Japanese Patent Laid-Open No. 2004-078209 in which the optical element is held by flexure members, such as leaf springs or elastic hinges, the surface shape of the optical element changes each time the optical element is removed and reattached.
In general, the flexure members have small manufacturing errors. Therefore, the flexure members hold the respective projections in such a state that the flexure members themselves are bent. When the optical element 210A is removed and then reattached, since the position of each projection is regulated only in two axial directions with respect to the corresponding holding member 220A, the relative position between the projection and the holding member 220A changes from the original relative position in axial directions other than the above-mentioned two axial directions. If the relative position between each projection and the corresponding holding member 220A changes when the optical element 210 is reattached, the manner in which each flexure member is bent also changes. As a result, force applied to the optical element 210A as the reactive force to the bending force also changes. Since this force serves to deform the optical element 210A, the surface shape of the optical element 210A changes in accordance with the change in the force.